Fish oil, a rich source of marine omega-3 polyunsaturated fatty acids (MO3PUFAs), is the most popular natural product used by U.S. adults. Substantial data support the beneficial effect of MO3PUFAs on colorectal cancer (CRC) prevention and treatment. However, the specific mechanisms through which MO3PUFAs influence CRC are not well understood. Increasing evidence supports a pivotal role of gut microbes in integrating dietary cues with host immunity and potentially mediating the anticancer effect of MO3PUFAs. Dietary fat composition is a major driver of the gut microbial community structure. Mice fed with a high-MO3PUFA diet demonstrate increased abundance of the gut bacteria that support the host immunoprotective system and improve the efficacy of cancer immunotherapy, such as Bifidobacterium and Lactobacillus genera, and decreased abundance of the microbes that dampen antitumor immunity, such as Fusobacterium nucleatum, which has been shown to promote CRC by generating a tumor-permissive microenvironment. Taken together with my recent findings that the anti-CRC effect of MO3PUFAs is related to the tumor immune microenvironment, these data aggregately support the hypothesis that MO3PUFAs modulate the gut microbial composition and function to shape the gut immune response and suppress CRC. To test this hypothesis, I will first leverage three large prospective cohort studies, the Nurses' Health Study (NHS) I and II and the Health Professionals Follow-up Study (HPFS), to characterize the gut microbial signature associated with long-term high intake of MO3PUFAs and assess whether certain microbes mediate the protective effect of MO3PUFAs on CRC incidence and survival. To establish causality, I will then bridge the observations to the clinic through a randomized controlled trial (RCT) in patients with prior adenoma by investigating the effect of MO3PUFA supplements on the gut microbiome, metabolome, and gene expression profiling in the colon. This innovative project will advance our understanding about the interplay between MO3PUFAs, gut microbiota, and host immunity in CRC. I am well suited to perform this research based on 1) my expertise in nutrition, epidemiology, quantitative methods, and biomarker research; 2) the exceptional multidisciplinary mentoring team comprised of leaders in their respective fields; and 3) the unparalleled research environment to support my career development. Through this study, I will expand my expertise in several new areas, including the gut microbiome, bioinformatics, and design and conduct of biomarker-based RCT. The proposed research and training will help achieve my long- term career goal to become an independent investigator, and develop a transdisciplinary research program in the gut microbiome and human nutrition for cancer prevention through integration of population-based epidemiologic study with patient-oriented clinical investigation. The findings yielded from my research will uncover new biological mechanisms relating diet to carcinogenesis, and identify novel biomarkers or targets that can be effectively translated into the clinic to improve cancer prevention and treatment.